1. Field of the Invention
This invention relates to an apparatus for biological treatment in which a water containing organic matter and/or nitrogenous compounds, such as waste water, is biologically treated. Specifically, a biological treatment apparatus of the attached-growth type in which biological treatment is effected under aerobic or anaerobic conditions by microorganisms growing on a medium packed in a vessel.
2. Description of the Prior Art
As apparatus for removing organic matter (e.g. BOD, COD, etc.) contained in waste water, etc. by making use of the metabolism of organisms or for nitrating or denitrificating nitrogenous compounds (e.g. NH.sub.4 -N, NO.sub.3 -N, etc.), biological treatment apparatus of the suspended-growth type and those of the attached-growth type are broadly known. The biological reactions are conducted by taking advantage of microorganisms inherently present in the water and waste (hereinafter referred to as "water") to be treated.
In the apparatus of the attached-growth type, there are four common species, i.e., trickling filters, roughing filters, rotating biological contactors and packed-bed reactors. Based on the mechanisms involved, the present apparatus is classified into the attached-growth type and may be similar to the packed-bed reactors. Thus, hereinafter, descriptions of the prior art will be focused on the packed bed reactors. Generally, in the packed-bed reactor, a bed of a packing medium such as gravel, macadom, artificial porous stones (e.g. sintered aggregate), anthracite, or plastics of various shapes including honeycomb shape, is provided in a vessel, through which bed the water containing organic matter, nitrogenous compounds, etc. is passed to effect biological decomposition/removal by the function of microorganisms (aerobic or anaerobic microorganisms) which are inherently present in the water and which settle and grow on the packing medium. Thus, in the apparatus of the packed-bed type, the concentration of microorganisms in the vessel can be made higher than in the biological treatment apparatus of the suspended-growth type, so that a high load treatment is possible for substances such as organic matter and nitrogenous compounds. Moreover, operation control is easy and the rate of removal can also be adjusted at will.
However, the apparatus of the packed-bed type has defects in that a continued treatment may cause thickening of organism films grown on the packing medium or clogging of the packing medium by the accumulation of suspended solids (referred to as "SS") from the water being treated, as a result of which a desired flow of water can not be secured and in the case of a treatment by aerobic microorganisms, the growth of anaerobic microorganisms occurs and the quality of the treated water is hence deteriorated. In consequence, periodical washing operation becomes necessary whereby washing water or washing gas is passed through the bed of packing medium to eliminate clogging of the medium.
The need for this washing entails the following problems. Namely, when granular materials such as gravel and macadom (having granular sizes of approx. 13 to 30 mm in general) are used as a packing medium in an apparatus of the packed-bed type, clogging of the medium is liable to occur due to its reduced void ratio, thus leading to an increased frequency of required washing. Further, the packing mediums of gravel, macadom and the like have relatively large specific gravities (generally approx. 2.2 to 2.6), so that it is difficult to fluidize the bed of the medium by passing washing water or washing gas therethrough and hence elimination of the clogging is not easy. On the other hand, in an apparatus of the packed-bed type using packing mediums of artificial stones, anthracite, etc., which have relatively small specific gravities (generally approx. 1.1 to 1.5), there is an increasing tendency of the packing mediums flowing out of the vessel, when the flow rate of washing fluid is increased for the purpose of securing sufficient washing effects. On the other hand, decreasing the washing flow rate results in insufficient washing and a prolonged time of washing. Thus, the apparatus involves problems in that their proper operation is not easy owing to large restrictions in washing operation and washing conditions in practical aspects.
Plastics formed in various shapes including honeycomb shape with large void ratios may also be used as the packing media in order to minimize their clogging and to permit their easy washing. However, a packing medium with a larger void ratio has a smaller surface area per unit volume (m.sup.2 /m.sup.3 : hereinafter simply referred to as "surface area") of the packing medium. Therefore, the area for growing microorganisms is smaller and hence the concentration of microorganism may not become high in the vessel, thus inviting the reduction of the efficiency of biological treatment.
In view of these facts, the applicant developed and proposed a novel apparatus for biological treatment using monofilaments as a carrier or a medium on which microorganisms can settle and grow, so that the above various problems were solved (Japanese Patent Laid-open No. 227189/1990).
The apparatus for biological treatment according to this proposal is characterized by the constitution in which a medium for microorganisms to settle and grow thereon made of bundles of long fiber as an aggregate of individual monofilaments are formed in a vessel in such a way that the lower ends of the bundles are fixed near the bottom of the vessel and the upper ends thereof are made free. Use of this apparatus not only increases significantly the concentration of microorganisms in the vessel but also minimize clogging of the medium. Even if clogging occurs, it can be eliminated by very simple washing.
Compared to the packed-bed reactors, a variety of merits are seen in the above-proposed apparatus of the attached-growth type, in which fiber bundles, whose lower ends are fixed and whose upper ends are made free, are used as a medium through which upward stream is passed to effect the treatment. However, it has been found that this apparatus leaves much to be desired if one wants to use it on a industrial scale.
Namely, in an industrial biological treatment facility, a very large quantity, e.g. several thousand m.sup.3 /day or more, of water must be treated. Therefore, quite a number of long fiber bundles must be disposed in the vessel to accommodate more microorganisms when an apparatus of the above style is constructed. As a result, enormous labor is needed in the disposal of a large number of the long fiber bundles in the vessel. Such labor should better be minimized to reduce the number of construction days and work costs.
Further, the apparatus, in which the upper ends of the long fiber bundles are made free, has an inherent shortcoming as described below, which has to be overcome. Namely, in an apparatus of the attached-growth type for biological treatment, a time-dependent variation takes place in the vessel in such a way that the amount of microorganisms growing on the medium, which is small in the beginning, increases gradually in accordance with the progress of the treatment. Even under such varying situations, however, it is necessary to maintain the condition under which the biological treatment by microorganisms is effected properly at all times. In the apparatus as proposed above, in which the upper ends of the long fiber bundles are made free, the long fiber bundles become gradually heavier with increasing amount of microorganisms growing thereon, leading to the tendency of settlement of all the bundles as a whole. However, when such settlement of the long fiber bundles once occurs, their density increases significantly in the lower section of the vessel with regard to its horizontal cross section. For example, in the treatment of a water containing relatively high concentrations of SS, etc., filtration takes place as the density of the fiber bundles in the lower section is increased, thus obstructing the inherent effective biological treatment by microorganisms. A filtration function adversely affects biological treatment because biological treatment requires passing large volumes of water.